Waste that contains iron values is generated at various locations of a steel plant during the processing of iron and steel. This waste material, which is commonly known as "revert," consists primarily of iron oxides. It is highly desirable that this waste be recycled to recover its iron value content, which may exceed 40 percent by weight. The dusts and sludges from the blast furnace and the basic oxygen furnace, and the sludges and mill scale from the rolling mill are particularly desirable for recycling purposes.
It is advantageous to convert steel mill waste into usable iron, inert slag and a relatively small amount of waste. This is not economically feasible unless all waste material that is generated in the plant is used, including oil laden sludge generated from flat rolling operations. Non-oily waste from the steel plant may be prepared in a sinter plant for use in a blast furnace. However, it is difficult to recycle oily steel mill waste. Steel mill waste that contains relatively large quantities of oils and greases cannot be recycled using sinter plants because volatilized oils and greases are not permitted for release to the atmosphere under EPA regulations and can cause fires in electrostatic precipitators used in the sinter plants.
Rolling mill scale when contaminated with oil from machinery is the primary source of oily waste in the steel mill. Such waste can be especially valuable in that it may contain about 60 to 75 percent by weight of iron. However, this residue is difficult to recycle due to its oil content. The oil in contaminated mill scale comprises about 10% of all the lubricants brought into a steel mill, and only about half of all oily sludge (contaminated and uncontaminated) can be processed by the sinter plant. Filippi, Removal of Organics from Recycled Materials, Conservation & Recycling, Vol. 8, No. 3/4, pp. 377-381, 1985.
In a hot strip rolling mill, a heated slab, sheet, bloom, billet or bar is passed between a series of rolls to reduce the thickness of the steel. In the rolling mill the hot steel is exposed to oxygen in the air as well as pressurized wash water on the entire surface of the steel as it is being rolled. This forms a layer of oxides on the steel, which must be removed before each successive rolling. Coarse scale removed at this stage is referred to as "mill scale."
As the steel is rolled in cold reduction a scale layer is mechanically broken away from the steel or is chemically removed by acids in a pickling process. The layer of scale is replaced by another layer every time the size or the shape of the steel is changed. When the scale is removed from the steel, it falls into a sewer in which high velocity water is flowing. Lubricating greases and oils from the rolling machinery and other mill debris join the water and scale material in the sewer to form an oily sludge.
Steel plants have disposed of oily steel mill waste, for example, by dumping it on-site in a controlled or regulated area or by shipping it to a waste site. Collection facilities have been used including settling pits and basins for removing oily waste from sludge so that water from the sludge can be reused or discharged. This conventional approach has limited prospects for reusing the oily waste and has created environmental problems in landfills.
Various methods have been proposed for recycling the oily sludge. For example, deoiling methods have been proposed that include a thermal kiln deoiling technique and a washing technique. The products of these processes are used in sintering operations. Solvent washing systems for removing the oil from the waste have also been proposed. Researchers are currently investigating the use of microwaves to separate oil from sludges. These methods are disadvantageous in that they have limited effectiveness, are generally not cost effective and may create new environmental problems.
Attempts have been made to recycle oily sludge by combining the oily sludge with sludges having substantially no oil or a low oil content. However, all of these attempts have been unsuccessful in mixing oily and substantially non-oily sludges together to form pellets of sizes amenable for use in various reduction processes.
To recover iron values from revert material, the material is agglomerated mechanically in a rotating disc or drum pelletizer in a manner known to those skilled in the art, in order to produce pellets of a particular size for the various reduction processes. The iron oxides in the pellets are then reduced to iron by one of a number of processes known to those skilled in the art.
One such reducing process commonly known as FASTMET uses a rotary hearth furnace to produce a direct reduced iron product. Coal is used as the reductant. The coal is added to the revert material to provide the reductant within the pellet. In another process known as cold bonded agglomeration, a bond for the revert material is provided by cement or a combination of cement and other material. In this agglomeration process no elevated temperatures are used, thus no lead, zinc, or other materials are eliminated from the revert. The agglomerated material is reduced in the blast furnace. Thus, the bond fails in the high temperature reducing atmosphere of the blast furnace.
Yet another example of a process used for reducing revert material is known as the INMETCO process. While the revert material normally contains carbon from the blast furnace waste, additional amounts of carbon in the form of coke may need to be added during the pelletizing process. The pellets are processed in a reducing atmosphere in a rotary hearth furnace for removal of tramp elements that are unwanted for iron and steel production, such as zinc, lead, chlorides and alkalis. The details of the INMETCO process are provided by the following publications, which are incorporated herein by reference in their entirety: Recycling of Iron and Steelworks Wastes Using the INMETCO Direct Reduction Process, Reprint from MPT-Metallurgical Plant and Technology, No. 4/1990; Pargeter et al., Ironmaking Using the INMETCO Process and Related Technologies, Ironmaking Proceedings, Vol. 44 1985; and Pargeter et al., Recycling of Waste and Flue Dust from the Steel Industry into Hot Metal Using the INMETCO Process, Proc. 44th Electric Steelmaking Conf., ISS-AIME Dallas, pp. 403-408 1986.
The INMETCO process will now be described by the following text from the article, Bauer et al., Recycling of Iron and Steelworks Wastes Using the Inmetco Direct Reduction Process, Reprint from MPT-Metallurgical Plant and Technology, No. 4, pp. 1-6 (1990).
"Recycling of high iron content iron and steelworks wastes to achieve reusable products is technically and economically feasible using the Inmetco direct reduction process. A solution achievable at relatively short notice is therefore now available for the urgent problems of waste management.
In this process, all oxidic dusts, sludges and oil-containing scale occurring in iron and steelworks are processed to green, carbon-containing pellets and then reduced to a high iron and low tramp element containing sponge iron at temperatures of around 1250.degree. C. in a rotary hearth furnace. Zinc, lead and cadmium are expelled nearly completely and converted to a highly-concentrated heavy metal containing secondary dust. These heavy metals can be recovered from this dust without any further concentration in the appropriate metallurgical processes. The hot sponge iron can be melted directly in a submerged arc furnace in its specific slag to achieve a low-sulphur hot metal suitable for charging into the LD steelmaking plant.
Iron-containing wastes occur in the production of hot metal and crude steel and in the processing of the crude steel to finished products. Such wastes comprise:
filter dusts and sludges from blast furnaces, steel plants and foundries; PA1 oil-containing scales from continuous casters and hot rolling mills; PA1 grinding dusts, sludges and pickling residues from steel treatment and processing; PA1 iron shot from slag separation. PA1 no thermal hardening of the pellets is necessary; PA1 the wustite stage is traversed very quickly. The principal cause of sticking is thus absent; PA1 the extremely high reduction temperatures result in correspondingly high degrees of metallization and practically complete evaporation of the heavy metals with exceptionally short reduction times, which, for their part, give rise to high specific throughputs; PA1 reduction conducted in a motionless bed avoids fines formation (abrasion). The waste-gas dust ("secondary dusts") thus contains the evaporated heavy metals at high concentration levels; PA1 the ash from the carbon sources cannot be removed from the product by magnetic separation. Therefore, it is not necessary to cool the sponge iron. Rather, the Inmetco product can be further processed `in a single heat` in an energy-saving fashion. The occurrence of char requiring dumping is avoided; PA1 the rotary hearth process is `thermally and chemically decoupled`. The heat supply in the gas space determines heating-up and reduction rates without the latter having a feed-back influence on heat transmission and thus on the further course of reduction. The ease of access to the process cycle offered by the motionless furnace and short processing times permit simple process monitoring and control.
These fine-grained dusts and sludges contain heavy metals, alkalis, sulphur compounds and, to a certain extent, oil. It is possible either only to a very limited extent or, not at all, to recycle them via the primary production processes, since their aggregate state and the impurities contained would impose on such primary processes burdens ranging up to complete impracticability. At present, the majority of such wastes is still dumped. In future, special hazardous waste landfills will become necessary, due to the environmentally harmful tramp elements contained, such as zinc, lead, cadmium, chromium (6+) and oil; such facilities, however, do not offer an acceptable solution in the long term. Recycling of such wastes back into the primary processes will thus become mandatory. It is, furthermore, rational in terms of materials recovery.
The Inmetco direct reduction process is a suitable method for the solution of this problem. It is currently in use for recycling and recovery of chromium and nickel from steelworks wastes produced in the stainless steel industry, but can also be universally applied for the reprocessing of bulk steelworks wastes.